Commander 2WD V8-4.7L (2008)
Tachometer: Description and Operation
Operation
OPERATION
The tachometer gives an indication to the vehicle operator of the engine speed. This gauge is controlled by the instrument cluster circuit board based
upon cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the Controller Area
Network (CAN) data bus.
The tachometer is driven by an electronic stepper motor unit that receives battery current on the instrument cluster electronic circuit board through the
fused ignition switch output (run-start) circuit whenever the ignition switch is in the ON or START positions. The cluster is programmed to move the
gauge needle back to the low end of the scale after the ignition switch is turned to the OFF position. The instrument cluster circuitry controls the gauge
needle position and provides the following features:
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Engine Speed Message - Each time the cluster receives an electronic engine speed message from the PCM it will calculate the correct engine
speed reading and position the gauge needle at that relative speed position on the gauge scale. The cluster will receive a new message and
reposition the gauge pointer accordingly about every 88 milliseconds. The gauge needle will continually be repositioned at the relative engine
speed position on the gauge scale until the engine stops running, or until the ignition switch is turned to the OFF position, whichever occurs first.
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Communication Error - If the cluster fails to receive a speed message, it will hold the gauge needle at the last indication for about three seconds,
or until the ignition switch is turned to the OFF position, whichever occurs first. After three seconds, the gauge needle will return to the left end of
the gauge scale.
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Actuator Test - Each time the cluster is put through the actuator test, the tachometer needle will be swept to several calibration points on the
gauge scale in a prescribed sequence in order to confirm the functionality of the gauge and the cluster control circuitry.
The PCM continually monitors the crankshaft position sensor to determine the engine speed. The PCM then sends the proper engine speed messages to
the ElectroMechanical Instrument Cluster (EMIC) (also known as the Cab Compartment Node/CCN).
For proper diagnosis of the crankshaft position sensor, the PCM, the EMIC, the CAN data bus or the electronic communication related to tachometer
operation a diagnostic scan tool is required. Refer to the appropriate diagnostic information.
